Localization of thalamic cells with tremor-frequency activity in Parkinson’s disease and essential tremor

One-pass deep brain stimulation of subthalamic nucleus and ventral intermediate nucleus for levodopa-resistant tremor-dominant Parkinson's disease

Objective

Tremor-dominant Parkinson's disease (TD-PD) can be further separated into levodopa-responsive and levodopa-resistant types, the latter being considered to have a different pathogenesis. Previous studies indicated that deep brain stimulation (DBS) of the subthalamic nucleus (STN) or the globus pallidus internus (GPi) individually was not sufficient for tremor control, especially for the levodopa-resistant TD-PD (LRTD-PD). The thalamic ventral intermediate nucleus (VIM) has been regarded as a potent DBS target for different kinds of tremors. Therefore, we focused on the LRTD-PD subgroup and performed one-pass combined DBSs of STN and VIM to treat refractory tremors, aiming to investigate the safety and effectiveness of this one-trajectory dual-target DBS scheme.

Methods

We retrospectively collected five LRTD-PD patients who underwent a one-pass combined DBS of STN and VIM via a trans-frontal approach. The targeting of VIM was achieved by probabilistic tractography. Changes in severity of symptoms (measured by the Unified Parkinson Disease Rating Scale part III, UPDRS-III), levodopa equivalent daily doses (LEDD), and disease-specific quality of life (measured by the 39-item Parkinson's Disease Questionnaire, PDQ-39) were evaluated.

Results

Three-dimensional reconstruction of electrodes illustrated that all leads were successfully implanted into predefined positions. The mean improvement rates (%) were 53 ± 6.2 (UPDRS-III), 82.6 ± 11.4 (tremor-related items of UPDRS), and 52.1 ± 11.4 (PDQ-39), respectively, with a mean follow-up of 11.4 months.

Conclusion

One-pass combined DBS of STN and VIM via the trans-frontal approach is an effective and safe strategy to alleviate symptoms for LRTD-PD patients.

Techniques, Indications, and Outcomes in Magnetic Resonance-guided Focused Ultrasound Thalamotomy for Tremor

Magnetic resonance (MR)-guided focused ultrasound surgery (MRgFUS) is the latest minimally invasive stereotactic procedure, and thalamotomy using this novel modality has demonstrated its effectiveness and safety, especially for patients with essential tremor (ET) and Parkinson's disease (PD). In Japan, the application of MRgFUS to treat ET and PD has recently been covered by health insurance. Technically, the transducer with 1024 elements emits ultrasound beams, which are then focused on the target with a phase control, resulting in optimal ablation by thermal coagulation. The technical advantages of MRgFUS are continuous intraoperative monitoring of clinical symptoms and MR images and fine adjustment of the target by the steering function. Postoperative tremor control is compatible with other modalities, although long-term follow-up is necessary. The adverse effects are usually transient and acceptable. Prognostic factors for good tremor control include high temperature and large lesion size. A high skull density ratio is a factor to achieve high temperature and large lesioning, but it may not be necessary and sufficient for clinical outcomes. For patients with advanced symptoms such as bilateral tremor or head/neck tremor, deep brain stimulation may be recommended because of the adjustability of stimulation and the possibility of bilateral treatment. Patients have high expectations of MRgFUS because of its non-invasiveness. To perform this treatment safely and effectively, physicians need to understand the technological aspects, the physiological principles. To choose the appropriate modality, physicians also should recognize the clinical advantages and disadvantages of MRgFUS compared to other modalities.

Increased Purkinje Cell Complex Spike and Deep Cerebellar Nucleus Synchrony as a Potential Basis for Syndromic Essential Tremor. A Review and Synthesis of the Literature

We review advances in understanding Purkinje cell (PC) complex spike (CS) physiology that suggest increased CS synchrony underlies syndromic essential tremor (ET). We searched PubMed for papers describing factors that affect CS synchrony or cerebellar circuits potentially related to tremor. Inferior olivary (IO) neurons are electrically coupled, with the degree of coupling controlled by excitatory and GABAergic inputs. Clusters of coupled IO neurons synchronize CSs within parasagittal bands via climbing fibers (Cfs). When motor cortex is stimulated in rats at varying frequencies, whisker movement occurs at ~10 Hz, correlated with synchronous CSs, indicating that the IO/CS oscillatory rhythm gates movement frequency. Intra-IO injection of the GABA_A receptor antagonist picrotoxin increases CS synchrony, increases whisker movement amplitude, and induces tremor. Harmaline and 5-HT_2a receptor activation also increase IO coupling and CS synchrony and induce tremor. The hotfoot17 mouse displays features found in ET brains, including cerebellar GluRδ2 deficiency and abnormal PC Cf innervation, with IO- and PC-dependent cerebellar oscillations and tremor likely due to enhanced CS synchrony. Heightened coupling within the IO oscillator leads, through its dynamic control of CS synchrony, to increased movement amplitude and, when sufficiently intense, action tremor. Increased CS synchrony secondary to aberrant Cf innervation of multiple PCs likely also underlies hotfoot17 tremor. Deep cerebellar nucleus (DCN) hypersynchrony may occur secondary to increased CS synchrony but might also occur from PC axonal terminal sprouting during partial PC loss. Through these combined mechanisms, increased CS/DCN synchrony may plausibly underlie syndromic ET.

Effects of ramped-frequency thalamic deep brain stimulation on tremor and activity of modeled neurons

OBJECTIVE

We conducted intraoperative measurements of tremor to quantify the effects of temporally patterned ramped-frequency DBS trains on tremor.

METHODS

Seven patterns of stimulation were tested in nine subjects with thalamic DBS for essential tremor: stimulation 'off', three ramped-frequency stimulation (RFS) trains from 130 → 50 Hz, 130 → 60 Hz, and 235 → 90 Hz, and three constant frequency stimulation (CFS) trains at 72, 82, and 130 Hz. The same patterns were applied to a computational model of the thalamic neural network.

RESULTS

Temporally patterned 130 → 60 Hz ramped-frequency trains suppressed tremor relative to stimulation 'off,' but 130 → 50 Hz, 130 → 60 Hz, and 235 → 90 Hz ramped-frequency trains were no more effective than constant frequency stimulation with the same mean interpulse interval (IPI). Computational modeling revealed that rhythmic burst-driver inputs to thalamus were masked during DBS, but long IPIs, concurrent with pauses in afferent cerebellar and cortical firing, allowed propagation of bursting activity. The mean firing rate of bursting-type model neurons as well as the firing pattern entropy of model neurons were both strongly correlated with tremor power across stimulation conditions.

CONCLUSION

Frequency-ramped DBS produced equivalent tremor suppression as constant frequency thalamic DBS. Tremor-related thalamic burst activity may result from burst-driver input, rather than by an intrinsic rebound mechanism.

SIGNIFICANCE

Ramping stimulation frequency may exacerbate thalamic burst firing by introducing consecutive pauses of increasing duration to the stimulation pattern.

Novel Functional MRI Task for Studying the Neural Correlates of Upper Limb Tremor

Introduction: Tremor of the upper limbs is a disabling symptom that is present during several neurological disorders and is currently without treatment. Functional MRI (fMRI) is an essential tool to investigate the pathophysiology of tremor and aid the development of treatment options. However, no adequately or standardized protocols for fMRI exists at present. Here we present a novel, online available fMRI task that could be used to assess the in vivo pathology of tremor.

Objective: This study aims to validate the tremor-evoking potential of the fMRI task in a small group of tremor patients outside the scanner and assess the reproducibility of the fMRI task related activation in healthy controls.

Methods: Twelve HCs were scanned at two time points (baseline and after 6-weeks). There were two runs of multi-band fMRI and the tasks included a “brick-breaker” joystick game. The game consisted of three conditions designed to control for most of the activation related to performing the task by contrasting the conditions: WATCH (look at the game without moving joystick), MOVE (rhythmic left/right movement of joystick without game), and PLAY (playing the game). Task fMRI was analyzed using FSL FEAT to determine clusters of activation during the different conditions. Maximum activation within the clusters was used to assess the ability to control for task related activation and reproducibility. Four tremor patients have been included to test ecological and construct validity of the joystick task by assessing tremor frequencies captured by the joystick.

Results: In HCs the game activated areas corresponding to motor, attention and visual areas. Most areas of activation by our game showed moderate to good reproducibility (intraclass correlation coefficient (ICC) 0.531–0.906) with only inferior parietal lobe activation showing poor reproducibility (ICC 0.446). Furthermore, the joystick captured significantly more tremulous movement in tremor patients compared to HCs (p = 0.01) during PLAY, but not during MOVE.

Conclusion: Validation of our novel task confirmed tremor-evoking potential and reproducibility analyses yielded acceptable results to continue further investigations into the pathophysiology of tremor. The use of this technique in studies with tremor patient will no doubt provide significant insights into the treatment options.

Short pauses in thalamic deep brain stimulation promote tremor and neuronal bursting

OBJECTIVE

We conducted intraoperative measurements of tremor during DBS containing short pauses (⩽50 ms) to determine if there is a minimum pause duration that preserves tremor suppression.

METHODS

Nine subjects with ET and thalamic DBS participated during IPG replacement surgery. Patterns of DBS included regular 130 Hz stimulation interrupted by 0, 15, 25 or 50 ms pauses. The same patterns were applied to a model of the thalamic network to quantify effects of pauses on activity of model neurons.

RESULTS

All patterns of DBS decreased tremor relative to 'off'. Patterns with pauses generated less tremor reduction than regular high frequency DBS. The model revealed that rhythmic burst-driver inputs to thalamus were masked during DBS, but pauses in stimulation allowed propagation of bursting activity. The mean firing rate of bursting-type model neurons as well as the firing pattern entropy of model neurons were both strongly correlated with tremor power across stimulation conditions.

CONCLUSIONS

The temporal pattern of stimulation influences the efficacy of thalamic DBS. Pauses in stimulation resulted in decreased tremor suppression indicating that masking of pathological bursting is a mechanism of thalamic DBS for tremor.

SIGNIFICANCE

Pauses in stimulation decreased the efficacy of open-loop DBS for suppression of tremor.

Tremor reduction and modeled neural activity during cycling thalamic deep brain stimulation

OBJECTIVE

The effectiveness of deep brain stimulation (DBS) depends on both the frequency and the temporal pattern of stimulation. We quantified responses to cycling DBS with constant frequency to determine if there was a critical on and/or off time for alleviating tremor.

METHODS

We measured postural tremor in 10 subjects with thalamic DBS and quantified neuronal entropy in a network model of Vim thalamic DBS. We tested 12 combinations of cycling on/off times that maintained the same average frequency of 125 Hz, four constant frequency settings, and baseline.

RESULTS

Tremor and neural firing pattern entropy decreased as the percent on time increased from 50% to 100%. Cycling with stimulation on for at least 60% of the time was as effective as regular stimulation. All cycling settings reduced the firing pattern entropy of model neurons from the no stimulation condition by regularizing pathological firing patterns, either through synaptically-mediated inhibition or axon excitation.

CONCLUSIONS

These results indicate that pauses present in cycling stimulation decreased its effectiveness in suppressing tremor, and that changes in the amount of tremor suppression were strongly correlated with changes in the firing pattern entropy of model neurons.

SIGNIFICANCE

Cycling stimulation may reduce power consumption during clinical DBS, and thereby increase the battery life of the implanted pulse generator.

Human pallidothalamic and cerebellothalamic tracts: anatomical basis for functional stereotactic neurosurgery

Anatomical knowledge of the structures to be targeted and of the circuitry involved is crucial in stereotactic functional neurosurgery. The present study was undertaken in the context of surgical treatment of motor disorders such as essential tremor (ET) and Parkinson's disease (PD) to precisely determine the course and three-dimensional stereotactic localisation of the cerebellothalamic and pallidothalamic tracts in the human brain. The course of the fibre tracts to the thalamus was traced in the subthalamic region using multiple staining procedures and their entrance into the thalamus determined according to our atlas of the human thalamus and basal ganglia [Morel (2007) Stereotactic atlas of the human thalamus and basal ganglia. Informa Healthcare Inc., New York]. Stereotactic three-dimensional coordinates were determined by sectioning thalamic and basal ganglia blocks parallel to stereotactic planes and, in two cases, by correlation with magnetic resonance images (MRI) from the same brains prior to sectioning. The major contributions of this study are to provide: (1) evidence that the bulks of the cerebellothalamic and pallidothalamic tracts are clearly separated up to their thalamic entrance, (2) stereotactic maps of the two tracts in the subthalamic region, (3) the possibility to discriminate between different subthalamic fibre tracts on the basis of immunohistochemical stainings, (4) correlations of histologically identified fibre tracts with high-resolution MRI, and (5) evaluation of the interindividual variability of the fibre systems in the subthalamic region. This study should provide an important basis for accurate stereotactic neurosurgical targeting of the subthalamic region in motor disorders such as PD and ET.

Amplitude- and frequency-dependent changes in neuronal regularity parallel changes in tremor With thalamic deep brain stimulation

The mechanisms by which deep brain stimulation (DBS) alleviates tremor remain unclear, but successful treatment can be achieved with properly selected frequency and amplitude. The clinical tremor response to thalamic DBS for essential tremor is dependent on the stimulation frequency and amplitude, and for high frequencies (> or = 90 Hz), increasing amplitude suppressed tremor, whereas for low frequencies (< 60 Hz), increasing amplitude aggravated tremor. We studied the effects of stimulation frequency and amplitude on the output of a population of intrinsically active model neurons to test the hypothesis that regularization of neuronal firing patterns is responsible for the clinical effectiveness of DBS. The firing patterns of model thalamocortical neurons were dependent on stimulation frequency and amplitude in a manner similar to the clinical tremor response. Above a critical frequency, increasing amplitude reduced the coefficient of variation (CV) of the neuronal firing pattern, whereas for low frequencies, increasing the amplitude increased the CV of neuronal activity. The correlation between the changes in tremor and the changes in the CV of neuronal firing supports the hypothesis that regularization of neuronal firing pattern during DBS is one of the mechanisms underlying the suppression of tremor.

Pulse-to-pulse changes in the frequency of deep brain stimulation affect tremor and modeled neuronal activity

The effectiveness of deep brain stimulation (DBS) in relieving the symptoms of movement disorders is dependent on the average frequency of stimulation. However, no one has yet examined whether the effectiveness of DBS in relieving tremor is dependent on the pulse-to-pulse (instantaneous) frequency of DBS. We examined the effects of paired-pulse thalamic DBS on tremor in subjects with essential tremor and on the firing of model neurons in a biophysically based computational model of DBS. DBS with an average rate of 130 Hz was more effective at reducing tremor when pulses were evenly spaced than when there were large differences between intrapair and interpair pulse intervals. Similar correlations were observed in the firing patterns of model neurons: increasing the difference between the intrapair and interpair intervals rendered model neurons more likely to fire synchronous bursts, more likely to fire irregularly, and less likely to entrain to the stimulus. The tremor responses provide evidence that the pulse-to-pulse frequency of DBS, not just its average rate, plays an important role in DBS function. Modeling results also suggest that effective DBS overrides oscillatory pathological activity and replaces it with more regularized neuronal firing patterns.

Difference in surgical strategies between thalamotomy and thalamic deep brain stimulation for tremor control

Stereotactic targeting strategies differ between thalamotomy and thalamic deep brain stimulation (DBS) for tremor control. In thalamotomy, a minimal radiofrequency lesion created within the lateral portion of the nucleus ventralis intermedius (Vim) often affords the best control of parkinsonian tremor, supporting the assumption that there is a concentrated cluster of cells within this area which is responsible for tremor. However, this assumption may not always be true; such neural elements sometimes appear to spread out across wide areas. Cells with tremor-frequency activity are widely distributed over the areas extending from the Vim to the nuclei ventralis oralis posterior and anterior (Vop and Voa). All of these cells appear to be more or less involved in tremor generation, especially in patients with essential tremor and post-stroke tremor. In contrast to radiofrequency lesions for thalamotomy, electrodes for DBS can be arranged in such a way that wide areas can be stimulated, if necessary. For this purpose, it is critically important to determine optimal placement and orientation of DBS leads for arranging the electrodes to yield maximal benefits in patients with tremor.